Internet Engineering Task Force (IETF) G. Nakibly
Request for Comments: 6324 NEWRSC
Category: Informational F. Templin
ISSN: 2070-1721 Boeing Research & Technology
August 2011
Routing Loop Attack Using IPv6 Automatic Tunnels:Problem Statement and Proposed Mitigations
Abstract
This document is concerned with security vulnerabilities in IPv6-in-
IPv4 automatic tunnels. These vulnerabilities allow an attacker to
take advantage of inconsistencies between the IPv4 routing state and
the IPv6 routing state. The attack forms a routing loop that can be
abused as a vehicle for traffic amplification to facilitate denial-
of-service (DoS) attacks. The first aim of this document is to
inform on this attack and its root causes. The second aim is to
present some possible mitigation measures. It should be noted that
at the time of this writing there are no known reports of malicious
attacks exploiting these vulnerabilities. Nonetheless, these
vulnerabilities can be activated by accidental misconfiguration.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6324.
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Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................22. A Detailed Description of the Attack ............................43. Proposed Mitigation Measures ....................................63.1. Verification of Endpoint Existence .........................63.1.1. Neighbor Cache Check ................................63.1.2. Known IPv4 Address Check ............................73.2. Operational Measures .......................................73.2.1. Avoiding a Shared IPv4 Link .........................73.2.2. A Single Border Router ..............................83.2.3. A Comprehensive List of Tunnel Routers ..............93.2.4. Avoidance of On-Link Prefixes .......................93.3. Destination and Source Address Checks .....................153.3.1. Known IPv6 Prefix Check ............................164. Recommendations ................................................175. Security Considerations ........................................176. Acknowledgments ................................................187. References .....................................................187.1. Normative References ......................................187.2. Informative References ....................................191. Introduction
IPv6-in-IPv4 tunnels are an essential part of many migration plans
for IPv6. They allow two IPv6 nodes to communicate over an IPv4-only
network. Automatic tunnels that assign IPv6 prefixes with stateless
address mapping properties (hereafter called "automatic tunnels") are
a category of tunnels in which a tunneled packet's egress IPv4
address is embedded within the destination IPv6 address of the
packet. An automatic tunnel's router is a router that respectively
encapsulates and decapsulates the IPv6 packets into and out of the
tunnel.
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Reference [USENIX09] pointed out the existence of a vulnerability in
the design of IPv6 automatic tunnels. Tunnel routers operate on the
implicit assumption that the destination address of an incoming IPv6
packet is always an address of a valid node that can be reached via
the tunnel. The assumption of path validity can introduce routing
loops as the inconsistency between the IPv4 routing state and the
IPv6 routing state allows a routing loop to be formed. Although
those loops will not trap normal data, they will catch traffic
targeted at addresses that have become unavailable, and misconfigured
traffic can enter the loop.
The looping vulnerability can be triggered accidentally, or exploited
maliciously by an attacker crafting a packet that is routed over a
tunnel to a node that is not associated with the packet's
destination. This node may forward the packet out of the tunnel to
the native IPv6 network. There, the packet is routed back to the
ingress point, which forwards it back into the tunnel. Consequently,
the packet loops in and out of the tunnel. The loop terminates only
when the Hop Limit field in the IPv6 header of the packet is
decremented to zero. This vulnerability can be abused as a vehicle
for traffic amplification to facilitate DoS attacks [RFC4732].
Without compensating security measures in place, all IPv6 automatic
tunnels that are based on protocol-41 encapsulation [RFC4213] are
vulnerable to such an attack, including the Intra-Site Automatic
Tunnel Addressing Protocol (ISATAP) [RFC5214], 6to4 [RFC3056], and
6rd (IPv6 Rapid Deployment on IPv4 Infrastructures) [RFC5969]. It
should be noted that this document does not consider non-protocol-41
encapsulation attacks. In particular, we do not address the Teredo
[RFC4380] attacks described in [USENIX09]. These attacks are
considered in [TEREDO-LOOPS].
The aim of this document is to shed light on the routing loop attack
and describe possible mitigation measures that should be considered
by operators of current IPv6 automatic tunnels and by designers of
future ones. We note that tunnels may be deployed in various
operational environments, e.g., service provider networks, enterprise
networks, etc. Specific issues related to the attack that are
derived from the operational environment are not considered in this
document.
Routing loops pose a risk to the stability of a network.
Furthermore, they provide an opening for denial-of-service attacks
that exploit the existence of the loop to increase the traffic load
in the network. Section 3 of this document discusses a number of
mitigation measures. The most desirable mitigation, however, is to
operate the network in such a way that routing loops cannot take
place (see Section 3.2).
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RFC 6324 Routing Loop Attack August 20112. A Detailed Description of the Attack
In this section, we shall denote an IPv6 address of a node by an IPv6
prefix assigned to the tunnel and an IPv4 address of the tunnel
endpoint, i.e., Addr(Prefix, IPv4). Note that the IPv4 address may
or may not be part of the prefix (depending on the specification of
the tunnel's protocol). The IPv6 address may be dependent on
additional bits in the interface ID; however, for our discussion
their exact value is not important.
The two victims of this attack are routers -- R1 and R2 -- that
service two different tunnel prefixes -- Prf1 and Prf2. Both routers
have the capability to forward IPv6 packets in and out of their
respective tunnels. The two tunnels need not be based on the same
tunnel protocol. The only condition is that the two tunnel protocols
be based on protocol-41 encapsulation. The IPv4 address of R1 is
IP1, while the prefix of its tunnel is Prf1. IP2 and Prf2 are the
respective values for R2. We assume that IP1 and IP2 belong to the
same address realm, i.e., they are either both public, or both
private and belong to the same internal network. The following
network diagram depicts the locations of the two routers. The
numbers indicate the packets of the attack and the path they
traverse, as described below.
[ Packet 1 ]
v6src = Addr(Prf1, IP2) [ Packet 2 ]
v6dst = Addr(Prf2, IP1) v6src = Addr(Prf1, IP2)
v4src = IP2; v4dst = IP1 +----------+ v6dst = Addr(Prf2, IP1)
//===========>| Router |-----------------\
|| | R1 | |
|| +----------+ v
.-. .-.
,-( _)-. ,-( _)-.
.-(_ IPv4 )-. .-(_ IPv6 )-.
(__ Network ) (__ Network )
`-(______)-' `-(______)-'
^^ |
|| +----------+ |
\\============| Router |<----------------/
[ Packet 1 ] | R2 | [ Packets 0 and 2 ]
v6src = Addr(Prf1, IP2) +----------+ v6src = Addr(Prf1, IP2)
v6dst = Addr(Prf2, IP1) v6dst = Addr(Prf2, IP1)
v4src = IP2; v4dst = IP1
Legend: ====> - tunneled IPv6, ---> - native IPv6
Figure 1: The Network Setting of the Attack
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The attack is initiated by an accidentally or maliciously produced
IPv6 packet (packet 0 in Figure 1) destined to a fictitious endpoint
that appears to be reached via Prf2 and has IP1 as its IPv4 address,
i.e., Addr(Prf2, IP1). The source address of the packet is an
address with Prf1 as the prefix and IP2 as the embedded IPv4 address,
i.e., Addr(Prf1, IP2). As the prefix of the destination address is
Prf2, the packet will be routed over the IPv6 network to R2.
R2 receives the packet through its IPv6 interface and forwards it
into the tunnel with an IPv4 header having a destination address
derived from the IPv6 destination, i.e., IP1. The source address is
the address of R2, i.e., IP2. The packet (packet 1 in Figure 1) is
routed over the IPv4 network to R1, which receives the packet on its
IPv4 interface. It processes the packet as a packet that originates
from one of the end nodes of Prf1.
Since the IPv4 source address corresponds to the IPv6 source address,
R1 will decapsulate the packet. Since the packet's IPv6 destination
is outside of Prf1, R1 will forward the packet onto a native IPv6
interface. The forwarded packet (packet 2 in Figure 1) is identical
to the original attack packet. Hence, it is routed back to R2, in
which the loop starts again. Note that the packet may not
necessarily be transported from R1 over the native IPv6 network. R1
may be connected to the IPv6 network through another tunnel.
The crux of the attack is as follows. The attacker exploits the fact
that R2 does not know that R1 does not configure addresses from Prf2
and that R1 does not know that R2 does not configure addresses from
Prf1. The IPv4 network acts as a shared link layer for the two
tunnels. Hence, the packet is repeatedly forwarded by both routers.
It is noted that the attack will fail when the IPv4 network cannot
transport packets between the tunnels, for example, when the two
routers belong to different IPv4 address realms or when ingress/
egress filtering is exercised between the routers.
The loop will stop when the Hop Limit field of the packet reaches
zero. After a single loop, the Hop Limit field is decreased by the
number of IPv6 routers on the path from R1 to R2. Therefore, the
number of loops is inversely proportional to the number of IPv6 hops
between R1 and R2.
The tunnels used by R1 and R2 may be any combination of automatic
tunnel types, e.g., ISATAP, 6to4, and 6rd. This has the exception
that both tunnels cannot be of type 6to4, since two 6to4 routers
share the same IPv6 prefix, i.e., there is only one 6to4 prefix
(2002::/16) in the Internet. For example, if the attack were to be
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launched on an ISATAP router (R1) and 6to4 relay (R2), then the
destination and source addresses of the attack packet would be
2002:IP1:* and Prf1::0200:5efe:IP2, respectively.
3. Proposed Mitigation Measures
This section presents some possible mitigation measures for the
attack described above. We shall discuss the advantages and
disadvantages of each measure.
The proposed measures fall under the following three categories:
o Verification of endpoint existence
o Operational measures
o Destination and source address checks
3.1. Verification of Endpoint Existence
The routing loop attack relies on the fact that a router does not
know whether there is an endpoint that can be reached via its tunnel
that has the source or destination address of the packet. This
category includes mitigation measures that aim to verify that there
is a node that participates in the tunnel and that its address
corresponds to the packet's destination or source addresses, as
appropriate.
3.1.1. Neighbor Cache Check
One way that the router can verify that an end host exists and can be
reached via the tunnel is by checking whether a valid entry exists
for it in the neighbor cache of the corresponding tunnel interface.
The neighbor cache entry can be populated through, e.g., an initial
reachability check, receipt of neighbor discovery messages,
administrative configuration, etc.
When the router has a packet to send to a potential tunnel host for
which there is no neighbor cache entry, it can perform an initial
reachability check on the packet's destination address, e.g., as
specified in the second paragraph of Section 8.4 of [RFC5214]. (The
router can similarly perform a "reverse reachability" check on the
packet's source address when it receives a packet from a potential
tunnel host for which there is no neighbor cache entry.) This
reachability check parallels the address resolution specifications in
Section 7.2 of [RFC4861], i.e., the router maintains a small queue of
packets waiting for reachability confirmation to complete. If
confirmation succeeds, the router discovers that a legitimate tunnel
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host responds to the address. Otherwise, the router discards
subsequent packets and returns ICMP destination unreachable
indications as specified in Section 7.2.2 of [RFC4861].
Note that this approach assumes that the neighbor cache will remain
coherent and not be subject to malicious attack, which must be
confirmed based on specific deployment scenarios. One possible way
for an attacker to subvert the neighbor cache is to send false
neighbor discovery messages with a spoofed source address.
3.1.2. Known IPv4 Address Check
Another approach that enables a router to verify that an end host
exists and can be reached via the tunnel is simply by pre-configuring
the router with the set of IPv4 addresses and prefixes that are
authorized to use the tunnel. Upon this configuration, the router
can perform the following simple checks:
o When the router forwards an IPv6 packet into the tunnel interface
with a destination address that matches an on-link prefix and that
embeds the IPv4 address IP1, it discards the packet if IP1 does
not belong to the configured list of IPv4 addresses.
o When the router receives an IPv6 packet on the tunnel's interface
with a source address that matches an on-link prefix and that
embeds the IPv4 address IP2, it discards the packet if IP2 does
not belong to the configured list of IPv4 addresses.
3.2. Operational Measures
The following measures can be taken by the network operator. Their
aim is to configure the network in such a way that the attacks cannot
take place.
3.2.1. Avoiding a Shared IPv4 Link
As noted above, the attack relies on having an IPv4 network as a
shared link layer between more than one tunnel. From this, the
following two mitigation measures arise:
3.2.1.1. Filtering IPv4 Protocol-41 Packets
In this measure, a tunnel router may drop all IPv4 protocol-41
packets received or sent over interfaces that are attached to an
untrusted IPv4 network. This will cut off any IPv4 network as a
shared link. This measure has the advantage of simplicity. However,
such a measure may not always be suitable for scenarios where IPv4
connectivity is essential on all interfaces. Most notably, filtering
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of IPv4 protocol-41 packets that belong to a 6to4 tunnel can have
adverse effects on unsuspecting users [RFC6343].
3.2.1.2. Operational Avoidance of Multiple Tunnels
This measure mitigates the attack by simply allowing for a single
IPv6 tunnel to operate in a bounded IPv4 network. For example, the
attack cannot take place in broadband home networks. In such cases,
there is a small home network having a single residential gateway
that serves as a tunnel router. A tunnel router is vulnerable to the
attack only if it has at least two interfaces with a path to the
Internet: a tunnel interface and a native IPv6 interface (as depicted
in Figure 1). However, a residential gateway usually has only a
single interface to the Internet; therefore, the attack cannot take
place. Moreover, if there are only one or a few tunnel routers in
the IPv4 network and all participate in the same tunnel, then there
is no opportunity for perpetuating the loop.
This approach has the advantage that it avoids the attack profile
altogether without need for explicit mitigations. However, it
requires careful configuration management, which may not be tenable
in large and/or unbounded IPv4 networks.
3.2.2. A Single Border Router
It is reasonable to assume that a tunnel router shall accept or
forward tunneled packets only over its tunnel interface. It is also
reasonable to assume that a tunnel router shall accept or forward
IPv6 packets only over its IPv6 interface. If these two interfaces
are physically different, then the network operator can mitigate the
attack by ensuring that the following condition holds: there is no
path between these two interfaces that does not go through the tunnel
router.
The above condition ensures that an encapsulated packet that is
transmitted over the tunnel interface will not get to another tunnel
router and from there to the IPv6 interface of the first router. The
condition also ensures the reverse direction, i.e., an IPv6 packet
that is transmitted over the IPv6 interface will not get to another
tunnel router and from there to the tunnel interface of the first
router. This condition is essentially translated to a scenario in
which the tunnel router is the only border router between the IPv6
network and the IPv4 network to which it is attached (as in the
broadband home network scenario mentioned above).
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RFC 6324 Routing Loop Attack August 20113.2.3. A Comprehensive List of Tunnel Routers
If a tunnel router can be configured with a comprehensive list of
IPv4 addresses of all other tunnel routers in the network, then the
router can use the list as a filter to discard any tunneled packets
coming from or destined to other routers. For example, a tunnel
router can use the network's ISATAP Potential Router List (PRL)
[RFC5214] as a filter as long as there is operational assurance that
all ISATAP routers are listed and that no other types of tunnel
routers are present in the network.
This measure parallels the one proposed for 6rd in [RFC5969] where
the 6rd Border Relay filters all known relay addresses of other
tunnels inside the ISP's network.
This measure is especially useful for intra-site tunneling
mechanisms, such as ISATAP and 6rd, since filtering can be exercised
on well-defined site borders. A specific ISATAP operational scenario
for which this mitigation applies is described in Section 3 of
[ISATAP-OPS].
3.2.4. Avoidance of On-Link Prefixes
The looping attack exploits the fact that a router is permitted to
assign non-link-local IPv6 prefixes on its tunnel interfaces, which
could cause it to send tunneled packets to other routers that do not
configure an address from the prefix. Therefore, if the router does
not assign non-link-local IPv6 prefixes on its tunnel interfaces,
there is no opportunity for it to initiate the loop. If the router
further ensures that the routing state is consistent for the packets
it receives on its tunnel interfaces, there is no opportunity for it
to propagate a loop initiated by a different router.
This mitigation measure is available only to ISATAP routers, since
the ISATAP stateless address mapping operates only on the Interface
Identifier portion of the IPv6 address, and not on the IPv6 prefix.
This measure is also only applicable on ISATAP links on which IPv4
source address spoofing is disabled. Finally, the measure is only
applicable on ISATAP links on which nodes support the Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) [RFC3315]. The following
sections discuss the operational configurations necessary to
implement the measure.
3.2.4.1. ISATAP Router Interface Types
ISATAP provides a Potential Router List (PRL) to further ensure a
loop-free topology. Routers that are members of the PRL for the site
configure their site-facing ISATAP interfaces as advertising router
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interfaces (see [RFC4861], Section 6.2.2), and therefore may send
Router Advertisement (RA) messages that include non-zero Router
Lifetimes. Routers that are not members of the PRL for the site
configure their site-facing ISATAP interfaces as non-advertising
router interfaces.
3.2.4.2. ISATAP Source Address Verification
ISATAP nodes employ the source address verification checks specified
in Section 7.3 of [RFC5214] as a prerequisite for decapsulation of
packets received on an ISATAP interface. To enable the on-link
prefix avoidance procedures outlined in this section, ISATAP nodes
must employ an additional source address verification check; namely,
the node also considers the outer IPv4 source address correct for the
inner IPv6 source address if:
o a forwarding table entry exists that lists the packet's IPv4
source address as the link-layer address corresponding to the
inner IPv6 source address via the ISATAP interface.
3.2.4.3. ISATAP Host Behavior
ISATAP hosts send Router Solicitation (RS) messages to obtain RA
messages from an advertising ISATAP router as specified in [RFC4861]
and [RFC5214]. When stateful address autoconfiguration services are
available, the host can acquire IPv6 addresses using DHCPv6
[RFC3315].
To acquire addresses, the host performs standard DHCPv6 exchanges
while mapping the IPv6 "All_DHCP_Relay_Agents_and_Servers" link-
scoped multicast address to the IPv4 address of the advertising
router. The host should also use DHCPv6 Authentication in
environments where authentication of the DHCPv6 exchanges is
required.
After the host receives IPv6 addresses, it assigns them to its ISATAP
interface and forwards any of its outbound IPv6 packets via the
advertising router as a default router. The advertising router in
turn maintains IPv6 forwarding table entries that list the IPv4
address of the host as the link-layer address of the delegated IPv6
addresses.
3.2.4.4. ISATAP Router Behavior
In many use case scenarios (e.g., enterprise networks, Mobile Ad Hoc
Networks (MANETs), etc.), advertising and non-advertising ISATAP
routers can engage in a proactive dynamic IPv6 routing protocol
(e.g., OSPFv3, the Routing Information Protocol Next Generation
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(RIPng), etc.) over their ISATAP interfaces so that IPv6 routing/
forwarding tables can be populated and standard IPv6 forwarding
between ISATAP routers can be used. In other scenarios (e.g., large
enterprise networks, etc.), this might be impractical due to scaling
issues. When a proactive dynamic routing protocol cannot be used,
non-advertising ISATAP routers send RS messages to obtain RA messages
from an advertising ISATAP router; i.e., they act as "hosts" on their
non-advertising ISATAP interfaces.
Non-advertising ISATAP routers can also acquire IPv6 prefixes, e.g.,
through the use of DHCPv6 Prefix Delegation [RFC3633] via an
advertising router in the same fashion as described above for host-
based DHCPv6 stateful address autoconfiguration. The advertising
router in turn maintains IPv6 forwarding table entries that list the
IPv4 address of the non-advertising router as the link-layer address
of the next hop toward the delegated IPv6 prefixes.
After the non-advertising router acquires IPv6 prefixes, it can
sub-delegate them to routers and links within its attached IPv6 edge
networks, then can forward any outbound IPv6 packets coming from its
edge networks via other ISATAP nodes on the link.
3.2.4.5. Reference Operational Scenario
Figure 2 depicts a reference ISATAP network topology for operational
avoidance of on-link non-link-local IPv6 prefixes. The scenario
shows two advertising ISATAP routers ('A', 'B'), two non-advertising
ISATAP routers ('C', 'E'), an ISATAP host ('G'), and three ordinary
IPv6 hosts ('D', 'F', 'H') in a typical deployment configuration:
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Non-advertising ISATAP router 'C' connects to one or more IPv6 edge
networks and also connects to the site via an IPv4 interface with
address 192.0.2.3, but it does not add the IPv4 address to the site's
PRL. 'C' next configures a non-advertising ISATAP router interface
with link-local address fe80::5efe:192.0.2.3, then receives the IPv6
prefix 2001:db8:0::/48 through a DHCPv6 prefix delegation exchange
via one of 'A' or 'B'. 'C' then engages in an IPv6 routing protocol
over its ISATAP interface and announces the delegated IPv6 prefix.
'C' finally sub-delegates the prefix to its attached edge networks,
where IPv6 host 'D' autoconfigures the address 2001:db8:0::1.
Non-advertising ISATAP router 'E' connects to the site, configures
its ISATAP interface, receives a DHCPv6 prefix delegation, and
engages in the IPv6 routing protocol the same as for router 'C'. In
particular, 'E' configures the IPv4 address 192.0.2.4, the ISATAP
link-local address fe80::5efe:192.0.2.4, and the delegated IPv6
prefix 2001:db8:1::/48. 'E' finally sub-delegates the prefix to its
attached edge networks, where IPv6 host 'F' autoconfigures IPv6
address 2001:db8:1::1.
ISATAP host 'G' connects to the site via an IPv4 interface with
address 192.0.2.5, and also configures an ISATAP host interface with
link-local address fe80::5efe:192.0.2.5 over the IPv4 interface. 'G'
next configures a default IPv6 route with next-hop address
fe80::5efe:192.0.2.2 via the ISATAP interface, then receives the IPv6
address 2001:db8:2::1 from a DHCPv6 address configuration exchange
via 'B'. When 'G' receives the IPv6 address, it assigns the address
to the ISATAP interface but does not assign a non-link-local IPv6
prefix to the interface.
Finally, IPv6 host 'H' connects to an IPv6 network outside of the
ISATAP domain. 'H' configures its IPv6 interface in a manner
specific to its attached IPv6 link, and autoconfigures the IPv6
address 2001:db8:3::1.
Following this autoconfiguration, when host 'D' has an IPv6 packet to
send to host 'F', it prepares the packet with source address
2001:db8:0::1 and destination address 2001:db8:1::1, then sends the
packet into the edge network where it will eventually be forwarded to
router 'C'. 'C' then uses ISATAP encapsulation to forward the packet
to router 'E', since it has discovered a route to 2001:db8:1::/48
with next hop 'E' via dynamic routing over the ISATAP interface.
Router 'E' finally forwards the packet to host 'F'.
In a second scenario, when 'D' has a packet to send to ISATAP host
'G', it prepares the packet with source address 2001:db8:0::1 and
destination address 2001:db8:2::1, then sends the packet into the
edge network where it will eventually be forwarded to router 'C' the
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same as above. 'C' then uses ISATAP encapsulation to forward the
packet to router 'A' (i.e., a router that advertises "default"),
which in turn forwards the packet to 'G'. Note that this operation
entails two hops across the ISATAP link (i.e., one from 'C' to 'A',
and a second from 'A' to 'G'). If 'G' also participates in the
dynamic IPv6 routing protocol, however, 'C' could instead forward the
packet directly to 'G' without involving 'A'.
In a third scenario, when 'D' has a packet to send to host 'H' in the
IPv6 Internet, the packet is forwarded to 'C' the same as above. 'C'
then forwards the packet to 'A', which forwards the packet into the
IPv6 Internet.
In a final scenario, when 'G' has a packet to send to host 'H' in the
IPv6 Internet, the packet is forwarded directly to 'B', which
forwards the packet into the IPv6 Internet.
3.2.4.6. Scaling Considerations
Figure 2 depicts an ISATAP network topology with only two advertising
ISATAP routers within the provider network. In order to support
larger numbers of non-advertising ISATAP routers and ISATAP hosts,
the provider network can deploy more advertising ISATAP routers to
support load balancing and generally shortest-path routing.
Such an arrangement requires that the advertising ISATAP routers
participate in an IPv6 routing protocol instance so that IPv6
address/prefix delegations can be mapped to the correct router. The
routing protocol instance can be configured as either a full mesh
topology involving all advertising ISATAP routers, or as a partial
mesh topology with each advertising ISATAP router associating with
one or more companion gateways. Each such companion gateway would in
turn participate in a full mesh between all companion gateways.
3.2.4.7. On-Demand Dynamic Routing
With respect to the reference operational scenario depicted in
Figure 2, there will be many use cases in which a proactive dynamic
IPv6 routing protocol cannot be used. For example, in large
enterprise network deployments it would be impractical for all
routers to engage in a common routing protocol instance, due to
scaling considerations.
In those cases, an on-demand routing capability can be enabled in
which ISATAP nodes send initial packets via an advertising ISATAP
router and receive redirection messages back. For example, when a
non-advertising ISATAP router 'B' has a packet to send to a host
located behind non-advertising ISATAP router 'D', it can send the
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initial packets via advertising router 'A', which will return
redirection messages to inform 'B' that 'D' is a better first hop.
Protocol details for this ISATAP redirection are specified in [AERO].
3.3. Destination and Source Address Checks
Tunnel routers can use a source address check mitigation measure when
they forward an IPv6 packet into a tunnel interface with an IPv6
source address that embeds one of the router's configured IPv4
addresses. Similarly, tunnel routers can use a destination address
check mitigation measure when they receive an IPv6 packet on a tunnel
interface with an IPv6 destination address that embeds one of the
router's configured IPv4 addresses. These checks should correspond
to both tunnels' IPv6 address formats, regardless of the type of
tunnel the router employs.
For example, if tunnel router R1 (of any tunnel protocol) forwards a
packet into a tunnel interface with an IPv6 source address that
matches the 6to4 prefix 2002:IP1::/48, the router discards the packet
if IP1 is one of its own IPv4 addresses. In a second example, if
tunnel router R2 receives an IPv6 packet on a tunnel interface with
an IPv6 destination address with an off-link prefix but with an
interface identifier that matches the ISATAP address suffix
::0200:5efe:IP2, the router discards the packet if IP2 is one of its
own IPv4 addresses.
Hence, a tunnel router can avoid the attack by performing the
following checks:
o When the router forwards an IPv6 packet into a tunnel interface,
it discards the packet if the IPv6 source address has an off-link
prefix but embeds one of the router's configured IPv4 addresses.
o When the router receives an IPv6 packet on a tunnel interface, it
discards the packet if the IPv6 destination address has an off-
link prefix but embeds one of the router's configured IPv4
addresses.
This approach has the advantage that no ancillary state is required,
since checking is through static lookup in the lists of IPv4 and IPv6
addresses belonging to the router. However, this approach has some
inherent limitations:
o The checks incur an overhead that is proportional to the number of
IPv4 addresses assigned to the router. If a router is assigned
many addresses, the additional processing overhead for each packet
may be considerable. Note that an unmitigated attack packet would
be repetitively processed by the router until the Hop Limit
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expires, which may require as many as 255 iterations. Hence, an
unmitigated attack will consume far more aggregate processing
overhead than per-packet address checks even if the router assigns
a large number of addresses.
o The checks should be performed for the IPv6 address formats of
every existing automatic IPv6 tunnel protocol (that uses
protocol-41 encapsulation). Hence, the checks must be updated as
new protocols are defined.
o Before the checks can be performed, the format of the address must
be recognized. There is no guarantee that this can be generally
done. For example, one cannot determine if an IPv6 address is a
6rd one; hence, the router would need to be configured with a list
of all applicable 6rd prefixes (which may be prohibitively large)
in order to unambiguously apply the checks.
o The checks cannot be performed if the embedded IPv4 address is a
private one [RFC1918], since it is ambiguous in scope. Namely,
the private address may be legitimately allocated to another node
in another routing region.
The last limitation may be relieved if the router has some
information that allows it to unambiguously determine the scope of
the address. The check in the following subsection is one example
for this.
3.3.1. Known IPv6 Prefix Check
A router may be configured with the full list of IPv6 subnet prefixes
assigned to the tunnels attached to its current IPv4 routing region.
In such a case, it can use the list to determine when static
destination and source address checks are possible. By keeping track
of the list of IPv6 prefixes assigned to the tunnels in the IPv4
routing region, a router can perform the following checks on an
address that embeds a private IPv4 address:
o When the router forwards an IPv6 packet into its tunnel with a
source address that embeds a private IPv4 address and matches an
IPv6 prefix in the prefix list, it determines whether the packet
should be discarded or forwarded by performing the source address
check specified in Section 3.3.
o When the router receives an IPv6 packet on its tunnel interface
with a destination address that embeds a private IPv4 address and
matches an IPv6 prefix in the prefix list, it determines whether
the packet should be discarded or forwarded by performing the
destination address check specified in Section 3.3.
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The disadvantage of this approach is that the administrative overhead
for maintaining the list of IPv6 subnet prefixes associated with an
IPv4 routing region may become unwieldy should that list be long
and/or frequently updated.
4. Recommendations
In light of the mitigation measures proposed above, we make the
following recommendations in decreasing order of importance:
1. When possible, it is recommended that the attacks be
operationally eliminated (as per the measures proposed in
Section 3.2).
2. For tunnel routers that keep a coherent and trusted neighbor
cache that includes all legitimate endpoints of the tunnel, we
recommend exercising the neighbor cache check.
3. For tunnel routers that can implement the Neighbor Reachability
Check, we recommend exercising it.
4. For tunnels having a small and static list of endpoints, we
recommend exercising the known IPv4 address check.
5. We generally do not recommend using the destination and source
address checks, since they cannot mitigate routing loops with 6rd
routers. Therefore, these checks should not be used alone unless
there is operational assurance that other measures are exercised
to prevent routing loops with 6rd routers.
As noted earlier, tunnels may be deployed in various operational
environments. There is a possibility that other mitigation measures
may be feasible in specific deployment scenarios. The above
recommendations are general and do not attempt to cover such
scenarios.
5. Security Considerations
This document aims at presenting possible solutions to the routing
loop attack that involves automatic tunnels' routers. It contains
various checks that aim to recognize and drop specific packets that
have strong potential to cause a routing loop. These checks do not
introduce new security threats.
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